1. Field of the Invention
The present invention relates to short-circuit detection circuits for detecting a short circuit of a signal line connected to a resolver and to resolver-digital converters having the short-circuit detection circuits.
2. Description of the Related Art
Resolvers are one type of sensors for detecting an angle. Resolvers include an exciter coil to which an exciting signal is sent and two detection coils for outputting two detection signals respectively, namely, sine-phase and cosine-phase signals, electromagnetically induced by the exciter coil; and outputs the two detection signals as signals indicating the rotation angle of the rotor. Either the exciter coil or the detection coils are fixed to the rotor and the other is located in the stator. The two detection signals output from the resolver are input to a resolver-digital (RD) converter, and the RD converter uses a tracking loop to calculate a digital value of the rotation angle of the rotor (the rotation angle of the rotor with respect to the stator).
The resolvers malfunction when a signal line connected thereto is short-circuited to the power supply (hereinafter called a short-circuit to power) or is short-circuited to the ground (hereinafter called a short circuit to ground). Japanese Patent Application Laid Open No. 2005-181186 discloses a method for detecting such a short circuit to power or to ground in a resolver. In this method, a short circuit to power or to ground is detected when each of the sums of squares of the sine-phase signal and the cosine-phase signal output from the resolver in response to an exciting signal sent to the resolver is smaller than a predetermined value.
Japanese Patent Application Laid Open No. 2005-24493 discloses a method for detecting a malfunction of a resolver based on a sine-wave signal or a cosine-wave signal obtained by detecting (operating to extract the envelope of a waveform) the output signal of the resolver. In this method, at lease one of the sine-wave signal and the cosine-wave signal is compared with a predetermined threshold to output a resultant rectangular-wave signal, and a malfunction of the resolver is detected according to the duty cycle or the period of the rectangular-wave signal.
The method disclosed in Japanese Patent Application Laid Open No. 2005-181186 cannot determine whether a short circuit has occurred in the signal lines for sending an exciting signal to the resolver or a short circuit has occurred in the signal lines for outputting the detection signals from the resolver. In addition, it cannot be determined whether the detected short circuit is a short circuit to power or to ground. As a result, it is difficult to find a portion causing the malfunction, and therefore, it is difficult to repair the resolver. Furthermore, it is not possible to detect a short circuit only in one of the signal lines for sending an exciting signal and the signal lines for outputting the detection signals, or to detect only one of a short circuit to power and a short circuit to ground.
In addition, in the method disclosed in Japanese Patent Application Laid Open No. 2005-181186, the sine-phase signal and the cosine-phase signal, which are analog signals, need to be squared in order to detect a short circuit, making the circuit configuration complicated.
The detection signals output from a resolver are obtained when the exciter coil receives an exciting signal and electromagnetically excites the detection coils. Therefore, the each detection signal before detection which is an operation to extract the envelope of a waveform includes the phase component of the exciting signal and the phase component corresponding to the rotation angle of the rotor. In the method disclosed in Japanese Patent Application Laid Open No. 2005-24493, the detection signal output from the resolver is detected to extract only the phase component corresponding to the rotation angle of the rotor, and the sine-wave signal or the cosine-wave signal, which has no phase component of the exciting signal, is used for detecting a malfunction of the resolver. Therefore, this method can only be applied to a case where the rotor of the resolver rotates in a regular manner. More specifically, with this method, a malfunction of the resolver cannot be detected if the rotor rotates in an irregular manner or if the rotor stops.